Because the air outside the car will change with different road conditions, such as a lot of dust, choking exhaust gas and so on. Most cars have air intake filters at the external circulation air intake, just like the filters of air conditioners at home, which are generally called pollen filters or dust filters.
Ventilation should adopt external circulation function. However, the warm air conditioning system has been used for a long time, and there are a lot of dust and bacteria on the air duct, warm air tank, air conditioner evaporator and air intake filter device, which makes the air blown out of the air outlet in the car unclean or smelly. This requires everyone to clean the heating and air conditioning system correctly to ensure the cleanliness of the ventilation in the car.